"""Flat-ROM checksum for the Triumph Keihin twin family -- reverse-engineered
and validated Aug 2026 (see ../WRITE_PATH.md for the full trace).

Source: `com.tuneecu.l.Ac(int, boolean)` in the decompiled TuneECU app,
called from MainActivity's map-info display ("Checksum : %04x", flagged
"*No-OEM"/"Error" on mismatch). This is a 16-bit sum-of-words checksum over
the calibration region of the flat ROM (0x50000-0x60000 for this ECU
family -- the exact same base/end this project's own `table_map.py` already
uses), stored in the last 2 bytes of that region.

Validated against 6 real, unmodified, official TuneECU downloads:
20187/20188/20191/20192/20262/20313 -- all computed == stored, exact.

One deliberate non-match, informative rather than a bug: the community
SAI/O2-delete file (20188Map2009AIRBOXBONNY.hex) has the exact same stored
checksum as its unmodified base (20188Map.hex) -- whoever built it edited
a few bytes by hand and never recomputed the checksum. TuneECU's own code
path for this looks like a *display/validity* check (flags "*No-OEM" /
"Error" in the UI) rather than a proven hard ECU-side write gate -- that
community file apparently worked for people despite the stale checksum,
which is consistent with this being an app-side sanity indicator rather
than (or in addition to) something the ECU's own bootloader independently
verifies during the real flash transfer. That ECU-side question is NOT
resolved by this -- see caveats below.
"""

from __future__ import annotations

CAL_BASE = 0x50000
CAL_END = 0x60000


def compute(rom: bytes, base: int = CAL_BASE, end: int = CAL_END) -> int:
    """16-bit sum-of-words checksum over rom[base:end-2], byte-swapped read
    (matches `l.Ac()`'s `bArr[pos ^ 1] | (bArr[pos] << 8)` exactly)."""
    length = (end - base) - 2
    total = 0
    i = 0
    while i < length:
        pos = base + i
        total += rom[pos ^ 1] | (rom[pos] << 8)
        i += 2
    return total & 0xFFFF


def stored(rom: bytes, end: int = CAL_END) -> int:
    return (rom[end - 2] << 8) | rom[end - 1]


def patch(rom: bytearray, base: int = CAL_BASE, end: int = CAL_END) -> None:
    """Recompute and write the checksum into rom[end-2:end] in place."""
    value = compute(bytes(rom), base, end)
    rom[end - 2] = (value >> 8) & 0xFF
    rom[end - 1] = value & 0xFF


if __name__ == "__main__":
    import sys

    from reconstruct_rom import flat_rom

    for path in sys.argv[1:] or ["20187Map.hex"]:
        rom = flat_rom(open(path, "rb").read())
        c, s = compute(rom), stored(rom)
        print(f"{path:40s} computed={c:04x} stored={s:04x} {'MATCH' if c == s else 'MISMATCH'}")